Because of their large starting current, squirrel-cage motors cannot always be started simply by closing a switch in the stator circuit, because the large current surge may generate a voltage swing that may cause e.g. disturbances in the operation of other electrical equipment. For this reason, different starting procedures are needed.
At present, squirrel-cage motors are often started using star-delta connection, in which the motor, whose normal connection mode is the delta connection, is first connected in the star mode and then changed to the delta connection mode. The star-delta connection is complex and expensive.
An asynchronous motor can also be started using a thyristor circuit as proposed in DE patent publication No. 2 718 604, in which thyristors are used. According to publication DE No. 2 721 287, a diode can be connected in parallel with the thyristor in such manner that the diode and the thyristor form an inverse-parallel circuit. The circuits proposed in the DE publication are complex.